In basic electronics and instrumentation, which of the following is NOT an analog device or quantity (i.e., not naturally varying continuously with amplitude over time)?

Introduction / Context:
Electronics deals with both analog and digital signals. Analog quantities vary continuously, whereas digital quantities assume discrete states. Identifying whether a device is analog or digital helps determine measurement techniques, noise considerations, and interface circuitry.

Given Data / Assumptions:

• Thermocouple generates a voltage proportional to temperature.
• Current in a circuit is a continuous physical quantity (amperes).
• A light switch is considered an on/off selector.
• A microphone converts sound pressure to a voltage that varies smoothly.

Concept / Approach:
An analog device produces or responds to signals that change smoothly over time. A digital device primarily has discrete states such as HIGH/LOW or ON/OFF. Decide for each option whether its primary behavior is continuous or discrete.

Step-by-Step Solution:
Thermocouple: output voltage varies continuously with temperature → analog.Circuit current: a continuous variable measured in amperes → analog.Audio microphone: outputs a voltage that follows air pressure over time → analog.Light switch: intended to be either closed or open (two discrete states) → digital behavior.

Verification / Alternative check:
Consider how you would measure each signal on an oscilloscope: the thermocouple, current (via shunt), and microphone all show smooth waveforms. The light switch shows two levels with transitions.

Why Other Options Are Wrong:
Thermocouple: analog voltage proportional to temperature.Current flow: continuous physical quantity.Audio microphone: analog transducer.Mercury thermometer: analog scale reading.

Common Pitfalls:
Confusing the installation of a switch in an analog circuit with the switch itself being analog. The device's primary operation is binary even if used within analog systems.

Final Answer:
Light switch